When confronted with something new, our brains instantly compare what we see, hear, feel, taste with all the 500 million previous experiences we had in our lives. This is why our intuition can say that we do not like/trust the person in front of us, without being able to say why. Apparently, we had bad experiences with these type of characters before somewhere, sometime.
The same is trie for the look and feel of a presentation. If it reminds us of boring experiences we had before, we switch off and anticipate a replay. A bullet point first slide, a stale clip art image, a cheesy stock photo, all tell-tale signs that what is about to follow is unlikely to be interesting.
There is a positive side to this as well: you can interest your audience, simply by being different. Even if different means that your slides are not very pretty.